Transparency in Images as planes, Blender 2.8
If you have a look at the Alpha channel (plug the Alpha output from the image into the Surface Node from the Material Output), you can see that alpha is stored as a black &amp; white image.
White means fully opaque, black stands for fully transparent parts. But values in between are also possible!

So if you want the opaque parts to be partially transparent, you need to dim the white color to a grayish color. There are many ways to do this. For instance, mix the alpha channel with black color using a MixRGB node. If the factor is 0, the result will be the original image whereas a factor of 1 results in a completely black image (which means fully transparent). 
Now that we have seen the effect this has, we can connect the mix output to the alpha input of the Principled BSDF. Then connect the BSDF output back to the material output. And there is the transparent image :)